1. Field of the Invention
The present invention relates to a voltage-pulse converting circuit that converts a voltage into a pulse signal by use of an integrator, and a charge control system incorporating the same.
2. Description of Related Art
Secondary batteries (batteries) such as a lithium cell have been used in a digital camera, a notebook PC, or the like. In this case, it is necessary to accurately control a battery power for the purpose of increasing a battery-based usable life as long as possible. To that end, a charge control system controlling the battery power has been generally provided together with a battery. In such charge control system, for example, a charger is connected between both ends of a battery charger completed by connecting plural secondary batteries in series. A current detecting resistor (sensing resistor) detecting charging/discharging current is connected to one terminal of the battery charger, and this sensing resistor converts the charging/discharging current into a voltage. A current accumulating unit (also referred to as “coulomb counter” or “current flow meter”) which receives the converted voltage is connected between both ends of the sensing resistor.
When the battery charger is connected with a charger (in case of charge), a current flows from a positive terminal to the secondary battery or the sensing resistor. On the other hand, if the battery charger is not connected with a charger (in case of discharge), a current flows out from the secondary battery.
That is, a current flows through the sensing resistor in opposite directions in accordance with the charging or discharging operation. Assuming that one terminal (on the battery charger side) of the sensing resistor is a CS positive terminal, and the other terminal is a CS negative terminal of the sensing resistor, the CS positive terminal has a positive potential in case of charge and has a negative potential in case of discharge. A voltage generated at the CS positive terminal is about several tens of mV.
As the above coulomb counter, an AD converter or voltage-frequency converting circuit (V-F converter) is used. Here, the V-F converter converts a voltage detected by the sensing resistor into a frequency pulse, and the pulses are added up by a counter to thereby evaluate the integral of the current based on a count value per unit time. According to this method, all circuits but the counter are analog circuits, and current amounts are continuously accumulated. Therefore, this method is advantageous in that a current conversion rate is high to enable accurate accumulation and a wide dynamic range as compared with the AD converter system.
Regarding the battery power control of the battery charger, the coulomb counter (hereinafter referred to as “V-F converter”) measures historical data of the charge/discharge current to grasp a current battery power of the battery charger.
In this example, if a coulomb counter intended to monitor charging/discharging of the battery charger (battery) is made up of the V-F converter, a voltage across the sensing resistor series-connected with the battery is converted into a pulse signal by use of the integrator, and the number of pulse signals is counted. Then, a current charge amount or discharge amount can be calculated based on the count value. At the same time, it is necessary to determine whether charge or discharge is executed depending on whether a positive or negative voltage is input to the V-F converter. Incidentally, it is difficult to determine whether the input voltage is a positive or negative voltage only by configuring the V-F converter. To that end, the circuit should be additionally provided with a charging pump and a comparator for determining whether the input voltage is a positive or negative voltage. Incidentally, it is possible to calculate an accumulated or released amount (mC) of charges per clock pulse based on the number or pulses and the result of determining whether charge or discharge is executed, and a current battery power of the battery charger can be derived from the count value of the pulses.
As an example of the above circuit, a voltage-frequency converter (V-F converter) used as a rechargeable battery monitor monitoring a charging current supplied to a rechargeable battery and a discharging current supplied from the rechargeable battery is disclosed in Japanese Patent Translation Publication No. 2001-520391 (Mader et al.).
FIG. 7 is a block diagram of the voltage-frequency converter disclosed by Mader et al. FIG. 8 shows an operational waveform thereof. In a V-F converter 224, one terminal of a battery is connected with a ground 220 of the system through a sensing resistor RS. A battery-side terminal of the sensing resistor RS (CS positive terminal) is connected with the battery. At the time of charging the battery, the sensing resistor RS flows from the CS positive terminal toward the CS negative terminal. In case of discharge, the sensing resistor RS flows from the CS negative terminal toward the CS positive terminal. A voltage across the sensing resistor RS is an input voltage of the V-F converter 224.
The V-F converter 224 includes an integrator, comparators 228 and 230 comparing an output voltage of the integrator with a reference voltage, and an interleaved bidirectional electric pump 238. The integrator includes a resistor Rint, a capacitor Cint having a capacity of, for example, 36 pF, and a differential amplifier 226. An output voltage (voltage at a node 222) VINT of the integrator lowers, that is, shows downward-sloping curve in FIG. 8 if an input voltage Vsc+ from the CS positive terminal is positive (a potential of the CS positive terminal is higher than that of CS negative terminal). On the other hand, if the input voltage Vsc+ from the input terminal CS− is negative (if a potential of the CS positive terminal is lower than that of the CS negative terminal), the output voltage increases, that is, shows an upward-sloping curve in FIG. 8.
The charging pump 238 includes charge pumping capacitors CP1 and CP2 having a capacitance of, for example, 12 pF. The charging pump 238 inverts a charge pumping direction to a node 240 in accordance with a charge pulse CHG or DSG. That is, in case of charge, charges corresponding to about ⅔ V of an output voltage of the integrator are pumped from the node 240 by alternately using the charge pumping capacitors CP1 and CP2 at the timing when the charge pulse CHG is supplied. On the other hand, in case of discharge, the charge pumping capacitors CP1 and CP2 are charged with a reference voltage VREF to alternately supply charges to the node 240 at the timing when the charge pulse DSG is supplied.
That is, if a positive voltage is input to the CS positive terminal of the integrator (in case of charge), in the V-F converting circuit 224, the comparator 230 compares the output voltage VINT of the integrator with a comparison voltage of 1 V, and the charge pulse CHG is output at a timing when the output voltage reaches 1 V. At this timing, charges of the node 240 are pumped. As a result, the output voltage VINT (node 222) of the integrator is returned to a positive value. Then, the output voltage VINT of the integrator is lowered again.
Meanwhile, if a negative voltage is input to the CS positive terminal of the integrator (in case of discharge), the comparator 228 compares the output voltage VINT of the integrator with a comparison voltage of 2 V, and the charge pulse DSG is output at a timing when the output voltage reaches 2 V. At this timing, charges are supplied to the node 240. The output voltage VINT of the integrator (node 222) is returned to a negative value. Then, the output voltage VINT of the integrator is increased.
In the case of configuring the battery monitoring coulomb counter (current flow meter), a voltage generated across the sensing resistor RS is inverted in accordance with the charging or discharging operation. Thus, in the case of measuring a current flow with a voltage-frequency converter using the integrator, it is necessary to determine whether the charge or discharge is executed as well as convert a frequency. In the V-F converter disclosed by Mader et al., an output voltage of the comparator 228 (charge pulse DSG) is used as a discharging clock representing a discharging operation and an output voltage of the comparator 230 (charge pulse CHG) is used as a charging clock representing a charging operation.
However, according to the method disclosed by Mader et al., it is necessary to pump or supply charges with the capacitor Cp1 or Cp2 of the charging pump 238 each time charge pulses are output from the comparators 228 and 230. The capacitors Cp1 and Cp2 are extra capacitors unlike a capacitor Cint that determines the original integration speed of the V-F converter. This results in a problem in that a chip size of an integrated circuit increases, and a cost increases.
In addition, each time the charge pulses CHG and DSG are output from the comparators 228 and 230, charges should be supplied/pumped to/from the node 240 through the charge/discharge of the capacitor Cp1 or Cp2 of the charging pump 238, which leads to a problem that current consumption increases.